1. Field of the Invention
This invention relates to solutions for etching copper in the production of printed wire boards. More particularly, this invention relates to additives for use with an alkaline ammoniacal cupric chloride etching bath which significantly increases the etching rate.
2. State of the Art
Printed wire boards (PWBs), also known as printed circuit boards, are generally manufactured by laminating copper foil onto a non-conductive substrate such as phenolic or epoxy-glass. A circuit is made by applying an etch resistant material to the copper foil in a pattern defining the circuit, and then subjecting the PWB to the action of an etching solution which dissolves all of the copper not covered by the etch resistant material.
Several different types of etching baths may be used. The most commonly used etching bath is alkaline ammoniacal cupric chloride, although an alkaline ammoniacal cupric sulfate bath is sometimes used. Each of these etching baths has advantages and disadvantages. Generally, the chloride baths have a higher etch rate than the sulfate baths. However, additives have been developed which can increase the etch rate of the sulfate baths by up to 100%. U.S. Pat. No. 4,784,785 to Cordani et al. discloses an alkaline ammoniacal copper sulfate etching bath including a mixture of an ammonium halide (preferably 4-5 g/L), a water-soluble salt containing sulfur, selenium or tellurium in the anion (preferably 0.004-0.01 g/L), an organic thio compound containing the group ##STR1## (preferably 0.004-0.01 g/L), and, optionally, a water-soluble salt of a noble metal such as silver (preferably 0.004-0.01 g/L). The sulfate etching bath developed by Cordani et al. has an etch rate almost twice as fast as previously used sulfate baths. Although this is a distinct improvement in the etch rate for the sulfate system, it is still one-half, or less, the rate of an ammoniacal copper chloride bath.
Chloride etching baths have also been improved by certain additives to increase the etching rate. U.S. Pat. No. 4,311,551 to Sykes teaches that the addition of cyanamide, or a cyanamide precursor such as thiourea, in amounts of 0.005-0.3 g/L, to an alkaline ammoniacal cupric chloride bath increases the etching rate by up to 38%. Given the higher etch rate of the chloride baths over the sulfate baths, this 38% increase is significant and chloride baths containing thiourea are the most commonly used today.
A conventional aqueous alkaline ammoniacal cupric chloride etching bath may contain the following ingredients:
______________________________________ 1.0-2.8 Moles/L Cupric ions as metallic copper 2.2-6.2 Moles/L Ammonium chloride 2.0-9.0 Moles/L Ammonium hydroxide 0.001-0.10 Moles/L Ammonium phosphate-di-basic q.s. to 1 liter Water 0.05 to 0.40 g/L Dithiobiurea or other additive. ______________________________________
The cupric ions (Cu.sup.++) are supplied in the etching solution by cupric salts such as cupric chloride, cupric nitrate, cupric acetate, etc. As the etching bath is used to dissolve copper, the resulting oxidized metallic copper and reduced cupric ions cause a buildup of cuprous ions (Cu.sup.+). These must be oxidized back to the cupric state. A replenisher solution containing ammonium hydroxide, ammonium salts and/or chelating agents and other ingredients is normally used to control the pH range of the system, to make up for the withdrawn complexing agents for the copper and other ingredients, and to dilute the copper concentration to an optimum level.
The use of thiourea as an additive in alkaline ammoniacal cupric chloride etchant baths has remained unquestioned in the industry even though the mechanism of the additive is not fully understood. However, it has recently been suggested that thiourea may be carcinogenic. There is therefore a need to find alternative means for increasing the etching rate of alkaline ammoniacal cupric chloride without using thiourea.